My 2002 Ford Taurus will shutter and die when running the air conditioner and sitting idle at a stop sign or stop light. When I turn on the air conditioner the rpms will fluctuate up high above 2 and then down below 1 then the car will die. If I turn off the air conditioner when idle the car revs fine no problem. I have replaced all spark plugs and wires, coil on cylinder 1, and EGR valve. My check engine light was on with codes PO401, PO171, PO174 and then found a loose vaccum hose. Fixed the hose, cleared the codes and the check engine light hasn't come back on.

The idle air control valve is responsible for engine idle.
this is a common failure on ford's. but I don't think yours is bad because it does idle high befor it dies.
you stated you had codes p0171 and p0174 these are lean codes for bank one and two, you said you found a vacuum leak and repaired it, the vacuum leak was mostly likely the cause of the lean codes, however the pcm has calibrated the fueling stategy for this lean mixture, what you need to do is erase the KAM "keep alive memory" this is done by unhooking the negative battery cable for 60 seconds, this is the only way to erase the KAM the $6000.00 machine I use wont even do it.

once the KAM is clean the computer will go into a relearn state and all operating values will be adjusted for the vehicles current condition, during this relearn session the vehicle may have some driveability issues (low or high idle, stalling, poor fuel economy) the relearn should complete within 10 minutes of idling and then 10 minutes of driving.

When the a/c comes on, the idle control is supposed to raise the idle to account for the load of the a/c being on.
If the idle air control does not raise the idle, the engine computer will kick up the idle momentarily to keep the car from stalling.
have your idle system checked

Tell us some more! Your answer needs to include more details to help people.You can't post answers that contain an email address.Please enter a valid email address.The email address entered is already associated to an account.Login to postPlease use English characters only.

Related Questions:

How does it idle in park , does it run ruff or smooth? Are any lights lit on the dash , check engine etc. ? Could have a partially plugged IAC air passage .idle air control (IAC) valve ! The engine computer uses the IAC to control idle , when under a load such as when put into gear the computer raises the idle ! It does this by allowing more air to by pass the closed throttle into the engine raising the idle ! If these air passages clog they restrict the air flow an cause a idle problem ! Now if it runs ruff at idle in park you could have vacuum leak ! Rough Idle How to Fix Ford Taurus Engine Stalls at Stop Signs

Idle Air Control Valve
Idle Air Control Valve Operation:A computer-controlled air bypass valve located in the throttle body. It helps control idle speeds by allowing air to bypass the throttle plates as to increase the idle spedle Speed Actuator
Idle Speed Actuator Operation:It regulates the position of the throttle valve plate(s) to compensate for additional loads on the engine, like the air conditioner. It measures engine RPM readings, compares them to the reference RPM in the engine-control computer memory and adjusts the idle air by-pass.ed.

sounds like a bad fuel pump...have this looked at by your mechanic...changed strainer and fuel filter at the same time..and if its been a while, the air filter too just to make sure that everything is breathing and working fine

your idle speed too low.take it to a dealer.they have to reset idle.it should be 600-800 rpms.when you turn on air conditioner.the computer steps up idling to keep engine from stalling .your curb idle too low.